Make the first line count. For a nonfiction introduction, avoid using words that lack impact, such as "This book is about all the different ways to cook broccoli." Rather, plunge your reader into the world of your book right away. "Broccoli haters, close this book right now and go get yourself a book about bacon."
For a fiction hook, use the first line to grab your reader, packing a lot of information into a few words. Use devices such as dialogue, internal monologue, deep description, dramatic contrast and careful word choice to enrich the first line. Descriptive details are less important. "He hated her the first time he saw her talking to his best friend, Jake," is worlds more intriguing than "He stepped into the room and saw the slim, blond woman talking to his best friend."
Give your reader a stylistic taste of what's to come. For fiction, write the beginning of the book using a vocabulary and sentence structure that sets the tone, mood and pace of the rest of the book. For example, write the beginning of a book rich in suspense using an understated tone teeming with deceptively quiet drama. "Feeling distinctly self-indulgent, Jane swallowed a caramel, then sipped her tea and gazed dreamily out the window. She straightened. Her hand shook only a little as she set down the cup."
Similarly, if you are writing nonfiction, begin your introduction using language that sets the tone for the book. Whether it's witty and satirical, conversational and informal, technical and detailed, or abstract and scholarly, make the flavor of the book clear from the beginning so your reader knows what to expect.
Give your reader something to wonder about. Before a reader gets too far into the beginning of a book, she wants to have a reason to read on. Raise a question in her mind. Why is the main character acting this way? How could there possibly be a thousand uses for flaxseed oil? Make sure your reader is intrigued enough by some question that she needs to read on.
Begin after the real beginning. Begin where it just begins to get interesting. Don't waste your reader's time by leading up to the interesting part. Entertain him now, or he won't wait around to be entertained.
Avoid boring your reader with dryness in a nonfiction introduction. Minimize your use of the passive tense. Banish extraneous words and phrases like "essentially" and "it is my considered opinion that." Trim your sentences down to their essence and use lively language to make your book easy to read and entertaining.